When it comes to adding vegetation to a scene in Poser you hit an obstacle because Poser doesn't support particles systems. As an example the amount of vegetation I added to this scene as particle systems in Blender would be impossible to do in Poser. Gallery image

Adding mesh object plants, trees, grass etc can quickly increase the scenes polycount by huge amounts. A single tree may have as many faces as your whole ampitheatre. Another option that you might consider is using planes with transparent PNG textures applied for your plants. In the attached screenshot from Blender the first panels show the simplicity of the actual mesh. The second panel shows the same model rendered in Poser with the transparency and shadows. A good result with minimal additional polycount.

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